A polarized capacitor has positive and negative terminals. In the design and fabrication of an electronic circuit, it is important to correctly orient each polarized capacitor. The result of misorienting a polarized capacitor can be damaging to the circuit and dangerous to individuals. First, the circuit failure due to the misoriented capacitor can be latent and unpredictable. Second, the resultant failure of the capacitor may be sudden and violent, causing physical damage to surrounding equipment and injury to personnel.
Heretofore, reliable determination of the orientation of a polarized capacitor by automatic means after connection of the capacitor into a circuit has been problematic. In general, no measurable parameter of the capacitor reliably indicates its orientation. While some polarized capacitors exhibit greater leakage current prior to failure when reverse-biased than when forward-biased, this phenomenon is not true of all polarized capacitors and is therefore not a reliable indicator of orientation. Also, for most circuits, only a limited amount of bias potential may be applied to a capacitor without having the measurement degraded by the effect of surrounding components in the circuit. Therefore, methods based on the leakage current are not universally applicable or reliable when performed in-circuit.
Another more reliable apparatus for determining the orientation of a polarized capacitor is disclosed in U.S. Pat. No. 5,159,526, assigned to the assignee of the present invention. The apparatus in U.S. Pat. No. 5,159,526 includes circuitry for applying first and second signals to the inner and outer conducting plates of a capacitor. These two signals are out of phase. The net field external to the capacitor as a result of these signals is of the same phase as the signal applied to the outer plate of the capacitor. This electric field is sensed and a third signal is generated in response thereto. The orientation of the capacitor with respect to the connected circuit is determined by inspecting the phase of the third signal. This apparatus has the simplicity of making only one measurement to determine the orientation of the capacitor, but has the disadvantage of not providing a very sensitive and accurate measurement. Instead, it gives a mere yes/no answer for the orientation of the capacitor depending on the phase of the third signal. Thus, it is not a very flexible method of determining the orientation.
Prior to the technique in U.S. Pat. No. 5,159,526, correct orientation of a capacitor could only be ensured by visual inspection, which is costly and error-prone.
Therefore, a need has arisen for a reliable method and apparatus for in-circuit determination of the orientation of a capacitor that provides a more sensitive, accurate and flexible measurement for making this determination.